For information on available APIs see here.
The OLA codebase is hosted on github: https://github.com/OpenLightingProject/ola . OLA developer documentation is at https://docs.openlighting.org/ola/doc/ . There are snapshots of specific releases and also a latest version which is automatically rebuilt from the git repo.
More in-depth knowledge of the internals of Open Lighting Architecture can be found at the OLA Wiki.